Pairing dynamics in strongly correlated superconductivity (original) (raw)
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Effects of phonon interaction on pairing in high-Tc superconductors
Physical Review B, 2008
We study the effects of phonon interaction on the superconducting pairing in the background of a d-wave gap, mediated by antiferromagnetic (AFM) spin fluctuations, using coupled BCS gap equations. We found that phonon interaction can induce a s-wave component to the d-wave gap in the (D+S) form with an interaction anisotropy and in the (D+iS) form without anisotropy, respectively. In either case, however, Tc is not enhanced compared to the pure d-wave pairing without phonon interaction. On the other hand, anisotropic phonon interaction can dramatically enhance the d-wave pairing itself and therefore Tc, together with the AFM spin fluctuation interaction. This (DAF M + D ph) type pairing exhibits strongly reduced isotope coefficient despite the large enhancement of Tc by phonon interaction. Finally, we study the combined type of (DAF M + D ph +iS)) gap and calculate the penetration depth and specific heat to be compared with the experiments.
? Pairing as a Mechanism of Superconductivity in Models of Strongly Correlated Electrons
Phys Rev Lett, 1995
We consider extended versions of the Hubbard model which contain additional interactions between nearest neighbours. In this letter we show that a large class of these models has a superconducting ground state in arbitrary dimensions. In some special cases we are able to find the complete phase diagram. The superconducting phase exist even for moderate repulsive values of the Hubbard interaction UUU.
Strength of the Spin-Fluctuation-Mediated Pairing Interaction in a High-Temperature Superconductor
2008
Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely. Here we analyze the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin-fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations and "kinks" in the fermionic band dispersions along the nodal direction. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.
Acta Physica Polonica Series B, 1998
The problem of phonon-induced and phonon-free superconductivity in the two-dimensional Hubbard model has been addressed. We have generalized the Eliashberg equations to account for both on-site and intersite pairing and consider the electron-electron and electron-phonon channels on an equal footing. This approach allows for the discussion of pairing and depairing properties of the local repulsive interaction. We demonstrate the possibility of cooperation between electron-phonon and electron-electron interaction in the stabilization of the d-wave superconductivity, in particular close to the experimental value of optimal doping (δ ≃ 0.15). We have also discussed the problem of phonon-induced superconductivity in the twodimensional Hubbard model close to the metal-insulator transition. Here, the Coulomb correlations have been incorporated within the Hubbard I approximation whereas the superconductivity is treated by the Eliashberg scheme. The results support the view that a d-wave component dominates in the gap function.
Acta Physica Polonica A
In this brief overview we discuss the principal features of real space pairing as expressed via corresponding low-energy (t-J or periodic Anderson-Kondo) effective Hamiltonian, as well as consider concrete properties of those unconventional superconductors. We also rise the basic question of statistical consistency within the so-called renormalized mean-field theory. In particular, we provide the phase diagrams encompassing the stable magnetic and superconducting states. We interpret real space pairing as correlated motion of fermion pair coupled by short-range exchange interaction of magnitude J comparable to the particle renormalized band energy ∼ tx, where x is the carrier number per site. We also discuss briefly the difference between the real-space and the paramagnon-mediated sources of superconductivity. The paper concentrates both on recent novel results obtained in our research group, as well as puts the theoretical concepts in a conceptual as well as historical perspective. No slave-bosons are required to formulate the present approach.
Effects of strong magnetic fields on pairing fluctuations in high-temperature superconductors
Physical Review B, 1999
We present the theory for the effects of superconducting pairing fluctuations on the nuclear spin-lattice relaxation rate 1/T 1 and the NMR Knight shift for layered superconductors in high magnetic fields. These results can be used to clarify the origin of the pseudogap in high-T c cuprates, which has been attributed to spin fluctuations as well as pairing fluctuations. We present theoretical results for s-wave and d-wave pairing fluctuations and show that recent experiments in optimally doped YBa 2 Cu 3 O 7Ϫ␦ are described by d-wave pairing fluctuations ͓V. F. Mitrović et al., Phys. Rev. Lett. 82, 2784 ͑1999͒; H. N. Bachman et al. ͑unpub-lished͔͒. In addition, we show that the orthorhombic distortion in YBa 2 Cu 3 O 7Ϫ␦ accounts for an experimentally observed discrepancy between 1/T 1 obtained by nuclear quadrupole resonance and nuclear magnetic resonance at low field. We propose an NMR experiment to distinguish a fluctuating s-wave order parameter from a fluctuating strongly anisotropic order parameter, which may be applied to the system Nd 2Ϫx Ce x CuO 4Ϫ␦ and possibly other layered superconductors. ͓S0163-1829͑99͒02818-0͔
A Spin Fluctuation Model for d-Wave Superconductivity
Superconductivity, 2008
We review the results of the spin-fermion model for correlated electron materials that are sufficiently close to an antiferromagnatic instability that their staggered static magnetic susceptibility in the normal state is large compared to that found in a conventional Fermi liquid. We demonstrate that for such materials magnetically-mediated superconductivity, brought about by the exchange of spin fluctuations, is a viable alternative to conventional phonon-mediated pairing, and leads to pairing in the d x 2 −y 2 channel. If the dominant interaction between quasiparticles is of electronic origin and, at energies much smaller than the fermionic bandwidth, can be viewed as being due to the emission and absorption of a collective, soft spin degree of freedom, the low-energy physics of these materials is accurately described by the spin-fermion model. The derived dynamic magnetic susceptibility and quasiparticle interaction coincide with the the phenomenonological forms used to fit NMR experiments and in earlier Eliashberg calculations. In discussing normal state properties, the pairing instability and superconducting properties, we focus our attention on those materials that, like the cuprate, organic, and some heavy electron superconductors, display quasi-two dimensional behavior. In the absence of superconductivity, at sufficiently low temperatures and energies, a nearly antiferromagnetic Fermi liquid is unconventional, in that the characteristic energy above which a Landau Fermi liquid description is no longer valid is not the Fermi energy, but is the much smaller spin-fluctuation energy,ω sf . For energies (or temperatures) between ω sf and the Fermi energy, the system behavior is quite different from that in a conventional Fermi liquid. Importantly, it is universal in that it is governed by just two input parameters -an effective spin-fermion interaction energy that sets the overall energy scale, and a dimensionless spin-fermion coupling constant that diverges at the antiferromagnetic quantum critical point. We discuss the pairing instability cased by the spin-fluctuation exchange, and "fingerprints" of a spin mediated pairing that are chiefly associated with the emergence of the resonance peak in the spin response of a d-wave superconductor. We identify these fingerprints in spectroscopic experiments on cuprateb superconconductors. We conclude with a discussion of open questions associated primarily with the nature of the pseudogap state found in underdoped cuprates.
Direct evidence for predominantly phonon-mediated pairing in high-temperature superconductors
The spectra of the second derivative of tunneling current d 2 I/dV 2 in the high-temperature superconductors YBa2Cu3O 7−δ and Bi2Sr2CaCu2O 8+δ show clear dip and peak features due to strong coupling to the bosonic modes mediating electron pairing. The energy positions of nearly all the peaks in −d 2 I/dV 2-like spectra match precisely with those in the phonon density of states obtained by inelastic neutron scattering. The results demonstrate that the bosonic modes mediating the electron pairing are phonons and that high-temperature superconductivity should arise primarily from strong coupling to multiple phonon modes.
Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:msubmml:miFeBmml:mn4
Physical review, 2018
We present an advanced method to study spin fluctuations in superconductors quantitatively, and entirely from first principles. This method can be generally applied to materials where electron-phonon coupling and spin fluctuations coexist. We employ it here to examine the recently synthesized superconductor iron tetraboride (FeB4) with experimental Tc ∼ 2.4 K [H. Gou et al., Phys. Rev. Lett. 111, 157002 (2013)]. We prove that FeB4 is particularly prone to ferromagnetic spin fluctuations due to the presence of iron, resulting in a large Stoner interaction strength, I = 1.5 eV, as calculated from first principles. The other important factor is its Fermi surface that consists of three separate sheets, among which two nested ellipsoids. The resulting susceptibility has a ferromagnetic peak around q = 0, from which we calculated the repulsive interaction between Cooper pair electrons using the random phase approximation. Subsequently, we combined the electron-phonon interaction calculated from first principles with the spin fluctuation interaction in fully anisotropic Eliashberg theory calculations. We show that the resulting superconducting gap spectrum is conventional, yet very strongly depleted due to coupling to the spin fluctuations. The critical temperature decreases from Tc = 41 K, if they are not taken into account, to Tc = 1.7 K, in good agreement with the experimental value.